In many applications it is important to be able to distinguish between light transmitted from different sources at receivers. In visible light positioning (VLP) this can be used to determine the direction of one or more light sources and so determine the position of the receiver. In visible light communication (VLC) or optical wireless communication (OWC) systems, different light sources may be transmitting different data streams and the receiver must be able to separate out these streams so that the data in each stream can be recovered.
To consider one particular scenario, by way of example, energy efficient light emitting diode (LED) based lighting is rapidly replacing conventional forms of interior lighting and as a result OWC using these LEDs as data transmitters is emerging as a promising method of indoor high speed wireless communication. Advantages of such arrangements include: simultaneous illumination and communication; no interference with existing RF systems; no licensing requirements; and very high signal to noise ratio (SNR).
In typical indoor scenarios, in order to provide adequate illumination throughout the room, LED lights are located at intervals on the ceiling. This provides an opportunity to use these lights as transmitters in a multiple input multiple output (MIMO) communication system. MIMO is a well-established technology in RF communications where it can provide multiplexing gains which increase the overall data rate.
However, many of the MIMO techniques developed for RF cannot be directly applied to OWC. In particular, unlike RF, the power of the received signal in OWC systems normally varies slowly with the position of the receiver. Thus, where a number of receivers are arranged in close proximity (as in a compact device) the received signal intensities from a given transmitter are very similar. Consequently, in OWC MIMO systems using spatially separated sources, the channel matrix has similar elements, and is therefore deficient, or nearly so. Common linear solution algorithms, including zero forcing and minimum mean square error, exhibit very poor performance in such cases. This prevents the transmitted signals from being decoupled in the receiver with acceptable output signal to noise ratios (SNRs), which degrades the system performance significantly.
One technique that has been explored in efforts to meet these requirements is to use a lens, or a configuration of multiple lenses, to separate light from different directions (i.e. directional, rather than spatial, diversity of receivers). Disadvantages of such arrangements include that they tend to be bulky and/or give insufficient separation of signals from different directions and/or have a small field of view.
In all of these applications, the receivers are normally integrated in portable handheld devices, e.g. smart phones, distributed within a given indoor scenario.
Taking into account all of the above considerations, it is desirable to provide a receiver having a large field of view such that light signals can be received at any receiver position, which has improved capability to distinguish signals originating from spatially distinct sources, and which is able to achieve these objectives within a compact structure. The present invention seeks to address these requirements.